1. Field of Invention
This invention relates to semiconductor manufacturing, and more particularly, to techniques for manipulating semiconductor wafers during fabrication and processing.
2. Description of Related Art
Modern semiconductor devices are typically created in large numbers on silicon wafers. A sequence of processes is performed on a wafer before it is sectioned into individual devices, which are then placed in packages with external leads or pins. Extreme precision is required in many stages of the manufacturing process, during which the wafers are generally handled by automated machinery. During critical processing steps, such as etching and implantation, the semiconductor wafer is mounted in a chuck. As defined herein, a “chuck” is any device to securely retain a wafer during a semiconductor fabrication step or steps. When these processes have been completed, the wafer must be removed and physically transported to the next manufacturing stage.
Wafers start out as circular discs of ultra-pure silicon. During the initial stages of manufacturing, the top surface of the wafer is subdivided into a matrix of small rectangular regions, each of which will become an individual integrated circuit. Then, after the many processing steps are completed, the wafer is sectioned along the boundaries of the matrix to separate the multiple integrated circuits on the wafer into “chips,” or “dies.”
A chuck is necessary to hold and support the wafer during semiconductor processing, and to prevent it from moving or flexing. Various manufacturing processes subject the wafer to extremely harsh conditions, such as vacuum, high temperatures, reactive gases and the application of a plasma. Throughout these processes, it is critical to maintain the position of the wafer. Originally, mechanical clamps were used to hold the wafer in the chuck during manufacturing. However, the periphery of the wafer, where it was retained by the clamps, could not be utilized. Furthermore, the use of clamps increases defects due to particles migrating from the clamps to the front side of the wafer. The clamps can also cause interactions between the areas of the wafer covered by the clamps at different layers, resulting in major defects in the wafer. Consequently, many semiconductor manufacturers have adopted the use of electrostatic chucks (ESCs), in which the attractive force between opposite electrical charges retains the wafer in the chuck. FIG. 1 illustrates the use of an ESC.
The semiconductor wafer 10 is dimensioned to fit within a circular chuck surface 12 on the top surface of the chuck 14. The chuck surface 12 can be made of an electrically-insulating material, and is typically equipped with holes 16 (or, sometimes, grooves) through which gas may be admitted to cool the wafer following a high temperature process. (Note that some processes run at higher temperatures, in which case, the gas and/or the chuck itself may be heated. In other processes, the wafer is cooled below room temperature for processing.) Lifter pins 18, which retract into the body of the chuck, are provided to raise and lower the wafer 10 onto the chuck surface 12 of the chuck. Note that the ESC allows the entire top surface of the wafer to be utilized for semiconductor fabrication.
FIG. 2 illustrates the principle of electrostatic adhesion in a mono-polar ESC. In this arrangement, the semiconductor wafer 10 is grounded, while the chuck surface 12 is connected to a DC voltage source 20. Within the chuck surface 12 is a single electrode 22, creating a region of positive charge 24 within the chuck surface 12. The formation of a charged region is possible due to the fact that the chuck surface is essentially non-conductive. Therefore, it is capable of accumulating an electric charge and of supporting a voltage gradient (i.e., a voltage difference can exist between two different points within the chuck surface). The wafer 10 is also non-conductive, and since the voltage source is ground-referenced, an oppositely-charged region 26 forms in the wafer. This establishes an attractive force between the wafer and the chuck surface, which binds the wafer in the chuck.
An alternative to the mono-polar chuck is the bi-polar ESC, shown in FIG. 3. The physical relationship of the wafer 10 to the chuck surface 12 of the chuck is the same as in the mono-polar ESC shown in FIG. 2. In this case, however, a pair of voltage sources 30a and 30b with opposite polarities are connected to two electrodes 32a and 32b. This arrangement creates separate positively charged 34a and negatively charged 34b regions within the chuck surface 12. The oppositely-charged regions within the chuck induce a complementary polarization within the wafer, forming a negatively-charged region 36a and positively-charged region 36b. As in the case of the mono-polar ESC, an electrostatic force attracts the wafer 10 to the chuck surface 12 of the chuck.
A problem exists with present electrostatic chucks, however. At the point in the process of manufacturing a wafer, when the procedures requiring extreme temperatures and pressures have been concluded, it is necessary to remove the wafer from the chuck. Before this can be done, however, it is necessary to remove the charge holding the wafer to the chuck. This is accomplished by applying a charge of opposite polarity to neutralize the existing charge. Once the electrostatic attractive force has been eliminated, the lifter pins (item 18 in FIG. 1) can raise the wafer off the chuck surface of the chuck, allowing it to be removed (typically, by a robotic arm) from the processing equipment. Neutralization of the accumulated electrical charge on the chuck and wafer is typically accomplished by reversing the polarity of the voltage source(s) for a prescribed time interval. But because of variations in the electrical properties of the wafer and the chuck surface, the length of time required for the accumulated charge to be neutralized is inconsistent and unpredictable.
Most semiconductor processing systems simply assign a fixed time for ESC charge neutralization. This is not a satisfactory solution, however. If the neutralization time is too short, a residual charge will remain to oppose removal of the wafer. If the neutralization time is too long, a reverse electrical charge may be induced in the wafer and chuck, reestablishing the attractive force. In either case, the lifter pins may attempt to remove the wafer while it is still attracted to the chuck surface of the chuck. This can cause the wafer to be violently ejected from the chuck, disrupting the manufacturing process and possibly damaging the wafer.